I was told that an Externship is just a label for a formalized program that offers learning and hands on experience to grad students (med students included) pre-degree. They can't call them internships because in medicine that has a different meaning.
For med students, usually it is the 3rd and 4th year students who might have time to be involved in externships. And depending on the program, some only want 3rd/4th year students.
I have never heard of a pre-med getting an externship without some other graduate degree (PhD, M.S.), but perhaps its possible.